1. Field of the Invention
This invention relates to document authentication, and in particular, it relates to a method for comparing documents employing Krawtchouk decomposition of image patches.
2. Description of Related Art
In situations where an original document, either in electronic form or in hardcopy form, is printed or copied to produce a hardcopy document, and the hardcopy document is distributed and circulated, there is often a need to determine whether a hardcopy document (referred to as the target document in this disclosure) is an authentic copy of the original, i.e., whether the hardcopy document has been altered while it was in circulation. A goal in many document authentication methods is to detect what the alterations (additions, deletions) are. Alternatively, some document authentication methods determine whether or not the document has been altered, without determining what the alterations are.
Various types of document authentication methods are known. One type of such method performs a digital image comparison of a scanned image of the target document with an image of the original document. In such a method, the image of the original document is stored in a storage device at the time of printing or copying. Alternatively, the image or data derived from the image may be stored in barcodes (in particular, two-dimensional (2d) barcodes) that are printed on the document itself or on the back of the sheet. Later, a target hardcopy document is scanned, and the stored image of the original document is retrieved from the storage device or the barcodes and compared with the scanned image of the target document. In addition, certain data representing or relating to the original document, such as a document ID, may also be stored in the storage device. The same data may be encoded in barcodes which are printed on the hardcopy document when the copy is made, and can be used to assist in document authentication.